PURPOSE: To provide an entanglement detector capable of automatically and continuously detecting an entanglement without requirement of a checkup man.

CONSTITUTION: This detector is equipped with plural light sources 10 and 10 arranged at different positions around a running test filament yarn 1 having entanglements 2 and capable of applying light to the filament yarn 1 from the direction crossing the traveling direction and with plural receptors 11 and 11 capable of receiving light applied from the light sources to the filament yarn. This detector is further equipped with a unit 16 for synthesizing output signals from the respective receptors, with a comparator 19 for comparing the synthesized signals with standard values N1, N2...Nn and with a judgment unit 20 for judging the entanglement strength of entanglement 2 based on the result of comparison. A yarn length measuring unit 22 and a calculation unit 21 are preferably incorporated so as to calculate the number of entanglements per unit length, the number of entanglements in each group separated by the entanglement strength and the mean distance value between entanglements. In addition, a transparent tube 30 is desirably arranged on a measuring filament yarn-running passage involving at least areas irradiated with the applied light so as to guide the filament yarn into the transparent tube while allowing it to run.